1. Field of the Invention
This invention relates to a nuclear fuel pellets to be loaded into an LWR (light water reactor), and more particularly to nuclear fuel pellets having high thermal conductivity and a method of manufacturing the same.
2. Description of the Prior Art
At present, in the LWR, there have been made plans for economical improvement, i.e., plans to achieve highly efficient combustion of nuclear fuel and to produce higher output therefrom.
Under such circumstances, main disadvantageous phenomena in the design of a fuel rod are as follows:
(1) a temperature increase in the center of the fuel rod;
(2) An increase in the discharge amount of gases generated on the nuclear fission in the fuel rod; and
(3) the interactions between the fuel pellets and the containing tube thereof.
In particular, (1) a temperature increase in the center of the fuel rod has most ,significantly adverse effects on other fuel behaviors.
A conventional nuclear fuel rod will be described with reference to a schematic configuration shown in FIG. 4. In FIG. 5, a conventional nuclear fuel rod 6 comprises nuclear fuel pellets 1, a containing tube 2 that contains the fuel pellets 1, an upper-end plug 3, a lower-end plug 4 and a plenum spring 5. The nuclear fuel pellets 1 are oxide pellets made of uranium oxide or mixed oxide, to which gadolinium oxide is added as a nuclear toxic substance.
Both the uranium oxide and the mixed oxide inherently have low thermal conductivity, and the addition of gadolinium oxide makes their thermal conductivities still lower. Thus, when the output of the fuel rod is increased, the temperature in the center of the fuel rod increases. As a result, the discharge amount of gases generated on the nuclear fission in the fuel rod is increased.
On the other hand, in order to enhance the mechanical strength of the nuclear fuel rod, high strength fibriform substances are uniformly dispersed in the ceramic that constitutes nuclear fuel pellets. This technique has been known to those skilled in the art (see Japanese Patent Laid-open Publication No.53-16198).
More specifically, metallic fiber and beryllium oxide fiber and whisker are used as high strength fibriform substances. Such fiber and whisker also have high thermal conductivity, so that they can serve to increase the thermal conductivity of the nuclear fuel pellets. However, the fiber and whisker are added to the ceramic, which constitutes the nuclear fuel pellets, in a dispersed fashion. Thus, the thermal conductivity of the nuclear fuel pellets cannot be increased significantly. As a result, the above-described disadvantages, which arise along with the output increase of the fuel rod, cannot be effectively reduced.
Further, another example has been disclosed in High Temperature-High pressures. Specifically, molybdenum metal is deposited in the grain boundaries of uranium oxide in order to increase the thermal conductivity of the nuclear fuel pellets. However, it is extremely difficult to manufacture such nuclear fuel pellets as indicated in this literature by use of an ordinary industrial technique.